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    /**
     * ES6诞生以前，异步编程的方法大概有下面四种
     *
     * 1.回调函数
     * 2.事件监听
     * 3.发布/订阅
     * 4.promise对象
     *
     * Generator函数将JavaScript异步编程代入了一个全新的阶段
    */
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